It is our intention to continue our parallel studies on the effect of aging on the regeneration of skeletal muscle. We will use the anterior tibialis muscle of the mouse as a source of material from which minced muscle will be prepared for either auto- or syngeneic implantation in the empty muscle bed. The rate of reabsorption of implant fragments and the rate of regeneration of functional muscle will be studied, with particular attention being paid to changes in the supporting structures and tissues. Both light and electron microscopic studies will be made at timed intervals after implantation in mice of chosen ages from 15 to 300 days. These studies will be paralleled by measurements of the rate of protein synthesis and degradation in an vitro preparation derived from the pectoral muscles of littermate mice. We will also attempt to measure the rate of RNA synethesis in the same animals. Besides providing data on overall rates of synthesis and degradation of protein and RNA which may be compared with the rate of muscle regeneration, these studies are expected to provide additional information as to the variations in compartmentalization of amino acids and RNA with age, a process which may be significant to control of muscle growth.